Applications calling for the measurement of light reflectance, or the use of light reflectance measurements, are generally faced with a wide selection of materials which can be used for standardization of reflection measurements. A variety of powders, paints, plastics, tiles, etc. are used for the purpose of establishing a standard by which other reflectance measurements can be gauged. Unfortunately, materials which have good initial reflectance characteristics are often unstable to light, are fragile and lack durability, are not cleanable, are expensive, are difficult to work with or to manufacture, age poorly, fail to reflect a high percentage of incident light, lack uniform reflectance over a desired wavelength range, decrease substantially in reflectance at shorter wavelengths, etc. and thereby fail to have the characteristics necessary to achieve a good reflectance standard which is usable as a standard over long periods of time. Visible light is only a part of the electromagnetic energy band. All sources of light, whether natural sunlight or artificial light, emit energy in the ultraviolet and in the infrared bands, as well as visible energy in the form of light. Upon exposure to electromagnetic energy photochemical damage can take place. Normally such damage takes two forms; fading or darkening of colors and structural damage caused by the breakdown of molecular bonds. This damage is not instantaneous, but takes place over long periods of time and is the cumulative effect of exposure. Irradiation by visible light in the 400 to 500 nanometer range primarily causes fading or darkening of colors in materials selected as reference standards although some minor structural damage may also occur. Irradiation by ultraviolet energy in the 300 to 400 nanometer range can cause both structural damage, i.e., embrittlement, loss of tear strength, etc. and fading and darkening of colors. These problems are especially acute in those industries where reflectance standards or reflective coatings must be used for comparison purposes in various measuring and testing devices such as spectrophotometers, colorimeters, and reflectometers.
Examples of materials currently used for reflectance standard purposes which give rise to the problems mentioned above include pigmented paint, magnesium oxide, magnesium carbonate, Halon, and barium sulphate. Pigmented paint, for instance, can result in low reflectance, give rise to light stability problems, and can crack or chip upon aging. The use of magnesium oxide and magnesium carbonate presents problems of light stability, aging and efficiency. Barium sulphate and Halon powder are presently the most commonly used reflectance materials and form almost ideal white standards. Unfortunately, since barium sulphate and Halon exist as powders they are very fragile. Even when the powders are pressed, the barium sulphate and Halon remain fragile. Moreover, the pressed barium sulphate and Halon lack reproducible reflectance characteristics and they are not cleanable once the pressed or compacted surface is soiled.